1. Field of the Invention:
This invention relates generally to diodes, and more particularly to improvements in diodes such as rectifying diodes which are suitable for operation with a relatively large current.
2. Description of the Prior Art:
As is well known, some of semiconductor devices mounted on a printed circuit board generate heat even under a normal operating condition. Particularly, rectifying diodes, which are designed to operate with a relatively large current, are known to generate heat. An example of the prior art rectifying diodes is shown in FIG. 4 of the accompanying drawings for the convenience of explanation.
The prior art rectifying diode 20 illustrated in FIG. 4 comprises a diode chip 21 interposed between a pair of leads 22, 23. Both leads have their respective inner ends 22a, 23a bonded to the chip in electrical and heat conduction therewith. The chip together with the lead inner ends is enclosed in a molded resin package 24 which is made of a hard resin such as an epoxy resin. The outer ends 22b, 23b of the respective leads are led out through an opposite pair of side walls 24a, 24b of the resin package, and bent to come into contact with the bottom wall 24c of the package.
The illustrated prior art diode, which is of the surface mounting type, is mounted on a printed circuit board with the bent lead ends bonded to relevant portions of the circuit conductor pattern. Under a normal operating condition, the diode generates heat, but such heat is allowed to effectively dissipate without inside heat accumulation because both of the leads 22, 23 are held in good heat conduction with the diode chip 21.
However, if an overcurrent passes through the diode, the heat dissipation provided by the leads 22, 23 can no longer catch up with heat generation, resulting in overheating of the diode. Such an overheat may lead to a fire or otherwise thermally damage the printed circuit board as a whole or adjacent heat sensitive semiconductor devices.
One way to eliminate the problem of overheat is to separately provide a protective fuse which is melt-cut upon passage of an overcurrent. Such an IC protective fuse is disclosed for example in U.S. Pat. No. 4,547,830 to Yamauchi. However, the provision of the separate IC protective fuse results in added cost and more importantly in a mounting space loss of the area-limited circuit board.
Another possible way to eliminate the overheat problem, though not actually practiced, is to modify the diode to have an additional function of preventing overcurrent. For this purpose, instead of sandwiching the diode chip 21 between the two leads 22, 23, one of the leads is separated apart from the chip and electrically connected thereto through a thin wire which is melt-cut upon passage of an overcurrent.
However, the above hypothetical solution is disadvantageous in that one lead, because of thermal separation from the chip, cannot be utilized for dissipating the heat, so that the diode may overheat even under a normal operating condition. Further, departure from the sandwiching arrangement necessitates a drastic modification of the existing manufacturing apparatus designed exclusively for producing sandwiching-type diodes.